Preparation of MoO3/MoS2-E composite for enhanced photoelectrocatalytic removal of antimony from petrochemical wastewaters

By doping of MoO3 to MoS2-E, MoO3/MoS2-E composite was produced to treat the antimony (Sb+5) from raw petrochemical industry wastewater. The effects of increasing MoO3/MoS2-E composite concentrations (0.01, 0.06, 0.50, 1.20, and 6 mg/L), times (5 min, 10 min, 20 min, 60 min, 80 min), and simulated sun light powers (2, 15, and 26 mW/m2) on the removal of Sb+5 was researched. According to X-ray diffraction (XRD), MoS2-E exhibited a pure hexagonal structure with peaks at 2Ɵ data of 15.56, 33.78, 40.59, and 61.43 cm−1 while MoO3 peaks showed similar configuration with the orthorhombic stage. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition. After Sb+5 removal, the additional MoO3 peaks were determined at 680, 967, and 997 cm−1. XPS spectra showed that after an oxidation period, “MoS2-E” was generated. Binding energy analysis showed that Mo5+ ions were produced from the partial transformation of MoO3. The MoO3 exhibited a vertical stacking on the MoS2-E. The filtered MoS2-E graph and relevant fast Fourier transfer pictures showed octahedral phase containing a proton. Field emission scanning electron microscopy analysis results showed that nano MoO3 exhibited a nanobelt structure. The maximum 10 mg/L Sb+5 removal was 93% at 1.20 mg/L MoO3/MoS2-E composite concentration at pH = 9 after 20 min at 15 mW/m2 simulated sunlight power via photoelectrocatalysis while the maximum Sb+5 removal via adsorption was detected as 80% for the same operational conditions in unilluminated conditions.

As aforementioned and summarized in a recent literature survey, it was observed that MoO 3 /MoS 2 nanocomposite was used extensively in the removal of some dyes and heavy metals in recent years. Although multiple data were obtained for MoO 3 / MoS 2 composite throughout removal of dyes and some pollutant removal in recent literature, no study undertaking the removal of Sb +5 using the MoO 3 /MoS 2 composite was found. Furthermore, no data was found about the adsorption and photocatalytic properties of the Sb +5 removal from a petrochemical industry wastewater with MoO 3 /MoS 2 nanocomposite. The effects of the sun light powers, pH, and ionic strengths on the removal of Sb +5 were not studied before using MoO 3 / MoS 2 composite.
The aim of the study was to treat the Sb +5 from a raw petrochemical industry wastewater via photoelectrochemical process. The effects of increasing MoO 3  , and SiO 3 2− for 0, 0.5, and 0.7 mg/L at pHs 6 and 9) on the removal of Sb +5 were investigated. Meanwhile, the effects of increasing MoO 3 composite concentrations (0.05, 0.08, 0.10, 0.15, 0.20, and 0.30 mg/L) in the MoO 3 /MoS 2 -E on the Sb +5 yields were researched. Furthermore, the adsorption of Sb+5 to MoO 3 /MoS 2 was also studied by using similar operational conditions in unilluminated conditions.

Preparation of MoS 2-E
MoS 2-E (Merck, Darmstadt-Germany) powder with particle sizes between 3 and 9 μm was used and was mixed with ethanol.

Preparation of MoO 3 /MoS 2-E composite
Firstly, 650 mg MoS 2-E (Merck, Darmstadt-Germany) and 38 mg MoO 3 (Merck, Darmstadt-Germany) were mixed into a 250 mL glass flask. It was located in a sonicator for 7 h. It was then centrifuged at 8000 rpm for 60 min in a Hatch Lange centrifuge (Dusseldorf-Germany, 2018). The supernatant water was collected and it was transferred into a supercritical Merck CO 2 incubator (Nüve, İstanbul-Turkey) with a heating jacket in a Gallemcamp autoclave (Roma-Italy). The autoclave was heated to 50 °C, and then CO 2 was charged into the desired pressure (8 MPa). Then, the dispersed volume was collected [18]. In order to minimize the cost of the MoO 3 /MoS 2-E composite prepared under laboratory conditions, the most appropriate cheap and native chemicals were used.

Reactor configuration for photoelectrocatalytic removal of Sb +5
A 5-L photoelectrocatalytical quartz glass rector (Merck, Darmstadt-Germany) was used in this study. The efficiency of the electrodes during simulated sunlight irradiation was evaluated in a device containing three electrodes under simulated sunlight powers varying between 2 Mw/m 2 and 26 mW/m 2 . Seven milligrams per liter sample was dispersed in methanol (Merck, Darmstadt-Germany) containing fluorine (Merck, Darmstadt-Germany) and suitable MoO 3 /MoS 2-E composite concentrations. The electrolyte was 0.3 M Na 2 SO 4 (Merck, Darmstadt-Germany). The known-control electrode was Ag/ AgCl (Gallemcamp, Roma-Italy) in saturated KCl (Merck, Darmstadt-Germany), and a Pt wire (Nüve, İstanbul-Turkey) was utilized for counter the electrode. The response was checked with simulated sunlight irradiation powers varying between 2 mW/m 2 and 26 mW/m 2 containing a KGS UV filter with a thickness of 2.5 mm (Sigma, St. Loius, Mo-ABD).
The Sb +5 photooxidation analysis was done in a 200 mL quartz glass reactor (Nüve, İstanbul-Turkey) using three electrodes with simulated sunlight containing a Horasan UV filter (Nüve, İstanbul-Turkey) at 460 nm at an OD of 0.89. The light power was adjusted to 2, 15, and 26 mW/m 2 to simulate four seasonal conditions. Thirty-five milligrams per liter sample was dispersed in 6 mL 95% ethanol (Merck, Darmstadt-Germany). This process was maintained in dark during 80 min to reach steady-state conditions during the adsorption of the electrode [18]. The samples containing Sb +5 were taken from a raw petrochemical industry wastewater. The nanocomposite diameters were measured under an advanced three ocular light microscope (Bushman-Biotar, Poznan-Poland)

Adsorption study
For Sb +5 adsorption analysis, quartz glass reactors with volumes of 400 mL were used. They were stirred continuously with a magnetic stirrer at a velocity of 1600 min -1 under unilluminated conditions at room temperature. The same operational conditions utilized in the photocatalytic reactor were applied to the adsorption reactor.

Measurement of Sb +5
The concentration of Sb +5 was determined using an Optima 7300 DV inductively coupled plasma-optical emission spectrometer (ICP-OES) to detect the correct concentrations of Sb +5 [19]. The plasma and the auxiliary gas flows were 15 L/min and 0.2 L/min, respectively. The instrument contained a Meinhard concentric pneumatic nebulizer and a cyclonic nebulizer chamber attached to a peristaltic pump. Both are used to introduce the samples into the plasma. An axial view and a spectral line of 220.353 nm were used for Sb +5 . For the preparation step, an ultrasonic bath was used. A 2 mL volume of a 10% HNO 3 solution (v/v) was added to the petrochemical industry wastewaters containing Sb +5 , and the samples were diluted to a final volume of 10 mL. The samples were then subjected to an ultrasonic bath for 20 min at 25 kHz and heated in a water bath for 1 h at 100 °C. Finally, the swabs were removed from the tubes, and the resulting solutions were analyzed by ICP OES. Data from the time-resolved ICP emission were accumulated during the construction phase of each chromatographic analysis. The values used for the eluent were 40 mM ethylendiaminetetraacetic acid (EDTA) at a flow rate of 1.0 mL/min. A 300 μL loop was used to manually inject the samples. The nebulizer flow was set to 0.7 L/min, reading time 1 s, RF power 1.2 kW. The accumulation of ICP-OES measurements was done in axial mode only to ensure the highest accuracy. The collected time-resolved emissions data was exported as text (CSV file), which was then integrated using the spreadsheet software Excel. Measurements of 1, 25, 50, 125, 250, 500, 1000, 2500, and 5000 mg/L were used to obtain the calibration curves. A linear model was chosen to fit the dependence of the total analyte count concentration. The limit of detection was stated as the concentration of analyte giving signals equivalent to three times the standard deviation of the blank plus the net blank intensity, for six independent replicates. Sb +5 recoveries for all analyses ranged from 98% to 99%.
Method detection limits (MDLs) were based on seven replicate measurements of a set of spiked calibration blanks. Analyte was added to each blank solution at concentrations estimated between 2 and 5 times the IDL. The MDL was calculated by multiplying the standard deviation of the seven replicate measurements by the appropriate Student's t-test value according to: MDL = (S) × (t) where s is the standard deviation and t is the Student's t-value, based on a 99% confidence level.
Both the Student's t-value and the standard deviation are based on n-1 degrees of freedom (t = 3.14 for six degrees of freedom). In order to establish the system performance, wastewater samples were measured along with appropriate standard certified reference materials (CRM).
Accuracy was calculated as the difference between the measured and certified concentrations for the CRM. The results are presented in the tables below. The accuracy and precision showed that the developed method performs well. The method detection limits calculated were generally in the low µg/L (ppb) range for a majority of elements. The reproducibility of the measurement was generally better than 2%. The analysis of spectral interference check solutions did not show any interference with any of the analytical lines selected.

Calculation of q e via adsorption and photocatalytic yields of Sb +5
Langmuir isotherm was used to calculate the Sb +5 amount attached on the surface of MoO 3 /MoS 2-E composite at equilibrium conditions, the concentration of adsorbed Sb +5 can also be described as follows [20]: q e = K q max C e / 1 + K C e (1) where q e and K are the concentration of adsorbed Sb +5 and the adsorption equilibrium constant, respectively. Q ax and C e are the maximum adsorption capacity and the concentration of the Sb +5 compound in the samples, respectively [20].
Eq. (1) can be rearranged into a linear form as shown below (Eq. 2): C e / q e = (1 / K q max ) + (C e / q max ) (2) A graph of C e / q e against C e would yield a straight line, which determines the adsorption capacity term, q max (mg/g), from the slope (1/q max ) as well as the adsorption equilibrium constant, K (L/mg), from the intercept (1/q max K). The adsorption terms q max and K relatively specify the tendency of the adsorbate to the surface of nanoadsorbent and explains the physical, chemical, and dynamic characteristic of a nanoadsorbent [20].

Validation of the methods
The data collected from the adsorption and photocatalytic studies and Sb +5 analyses were performed in triplicate samples with standard uncertainties and standard deviations. The data given in all figures and tables were the mean values of these data. Two control reactor for photocatalytic and adsorption studies were operated without Sb +5 . The MoO 3 /MoS 2 composite were produced serially in the same time to prevent the possible differences between nanocomposites.

X-ray diffraction (XRD) and Raman spectra of the MoS 2-E , MoO 3 and MoO 3 /MoS 2-E composite
The XRD patterns of the sample, MoS 2-E , and MoO 3 in wastewater are given in Figure 1a, while the Raman spectra of the sample are illustrated in Figure 1b. The peaks of the initial MoO 3 in wastewater exhibited similar configuration with the orthorhombic stage of MoO 3 (JCPSs no. 04-0509) [21]. During the treatment of Sb +5 in petrochemical wastewater, after MoS 2-E utilization, it was found that MoO 3 showed a major crystal phase of hexagonal MoO 3 in the MoS 2-E containing samples (JCPDs no. 22-0789). The differential peaks of this hexagonal MoO 3 exhibited 2Ɵ values varying between 18.19° and 21.49° (Figure 1a) [22].
In Figure 1a, it is observed that MoS 2-E was generated and their patterns exhibited similarities with the XRD pattern. All different peaks in the XRD pathway can exactly show similarities with a pure hexagonal MoS 2-E stage (JCPDS NO. 38-1586). The major differential peaks at 2Ɵ data of 15.56, 33.78, 40.59, and 61.43 cm -1 can be attributed to reflections numbered (005), (109), (111), and (119), respectively. It is important to note that other special peaks were not detected. This showed that the produced composite exhibited high purity. Furthermore, differences in hills were detected since the crystal structure of the MoS 2-E changed [23,24]. After Sb +5 removal, the additional MoO 3 peaks determined at 680, 967, and 997 cm −1 as reported by Kumar et al. [25]i.e., MoO3-II, was a result of the topotactic phase transformation of hexagonal-MoO3 (h-MoO3. The presence of hills was found generally at 298, 696, 8 4 5 , and 1 0 0 2 cm −1 in this study. Furthermore, a novel Raman hill was detected at 232 cm −1 (Figure 1b). This can be explained by the presence of the photons produced by MoS 2-E . As a result, two protons converted in a photon-containing step. Therefore, MoS 2-E transformed to pristine form ( Figure 1b) [18]. This can be explained by the presence of the photons produced by MoO 3 /MoS 2-E . As a result, two protons converted in a photon-containing step. Therefore, MoO 3 /MoS 2-E transformed to pristine form ( Figure 1b).

X-ray photoelectron spectroscopy (XPS) analysis
C, Si, S, Mo, and O elements were found when XPS scanning of MoS 2-E was performed ( Figure 2). The obtained Si 2p spectrum showed that binding energy was found at 103.3 eV for SiO 2 [26]. The values found showed that the C, Si, and partial O elements were formed due to SiO 2 and there were OH radicals absorbed on the MoO 3 /MoS 2-E surface. Figure 3a shows the spectra of XPS Mo 3d, S 2p and O 1s given from MoS 2 exposed to the photocatalytic mechanism. In the Mo 3d spectra, three peaks were detected as reported by Qi et al. and Yin et al. [27,28]. These were Mo (IV) and Mo (VI) doublets and they corresponded to the Mo (VI) 3d 5/2 at ∼228.9 eV, Mo (V) 3d 3/2 at ∼236.8 eV, and a sum of Mo (VI) 3d 3/2 and Mo (IV) 3d 5/2 at ∼231.9 eV were detected. Based on the position, area, and width of both Mo (VI) 3d 5/2 and Mo (VI) 3d 3/2 peaks, the Mo 3d spectrum was fitted to the two Mo (VI) and Mo (IV) peaks (Mo 3d 5/2 and Mo3d 3/2 : 231.6, 236.5 eV; and 228.4, 231.6 eV).
The contribution of Mo 3d varied between 40% and 51% which is corresponding to MoO 3 and MoS x O y , respectively [29,30]. Two peaks were detected from the S 2p XPS spectra (Figure 3b). These separately corresponded exactly to MoS x O y at 163.0 eV and to sulfuric acid at 169.1 eV as reported by Luther et al. and Manthiram and Alivisatos [31,32].
The O 1s spectra shown a broad and asymmetric peak which should be a superposition of O 1s peaks with different chemical structures such as MoO 3 , MoS x O y , absorbed OH radicals and sulfonated contaminants ( Figure 3c) [33,34]. Figure 4 shows the XPS profile of the MoS 2-E before being exposed to photoelectrocatalytic operation. The higher O and lower Mo concentrations were obtained at the surface, after which they were stable [35].
Previous studies showed that [34,36] SO and SO 2 were already produced in the MoS 2 as volatile products as a result of photocatalytic irradiation, and S losses was found [29,36]. In this study, any significant variations not found in S concentration from the XPS survey of the MoS 2-E exposed to the photoelectrocatalytic operation. The element compositions obtained from the XPS survey spectrum showed that Mo, S, and O contents (with percentages of 26%, 35% and 39% respectively), normalized to 100% ( Figure 5).

High-resolution transmission electron microscopy (HRTEM) analysis
HRTEM was utilized to assess the morphology, the heterostructure and the lattice arrangement of the MoO 3 /MoS 2-E composite. A typical HRTEM image is shown in Figure 6a. The MoO 3 exhibited nanostructure which has vertical stacking on the nanocomposite of MoS 2-E . The filtered MoS 2-E graph and relevant fast Fourier transfer (FFT) picture is illustrated in Figure 6b. This was a trigonal (octahedral) phase of the MoS 2-E containing a proton in the comparison to the trigonal prismatic phase of the MoS 2-E containing two protons [27].

Energy-dispersive X-ray spectroscopy (EDS) analysis
The EDS line scan of MoO 3 /MoS 2-E composite was given in Figure 7. As can be seen, the nanocomposite contains S, Mo, and O. The atomic percentages were clearly presented in Figure 8. It was shown that EDS analysis exhibited similarities with the data obtained from the XPS scan profile. The atomic percentages of Mo, S, and O obtained from Figure 8 were accounted as 26.3%, 34.9%, and 38.8% with a total score of 100%. These studies exhibit similarities with the study performed by Acerce et al. [37].

Field emission scanning electron microscopy (FESEM) analysis
The structure of the MoO 3 and MoS 2-E nanoparticles produced under laboratory were investigated by the FESEM Analysis (Figures 9a and 9b). In general, the produced MoO 3 nano material exhibited some nanobelt structure with a length of 3 nm (Figure 9a) [38].

Effect of MoO 3 /MoS 2-E concentrations on the removal of Sb +5 concentration via adsorption
The preliminary studies showed that the maximum adsorption yields of 10 mg/L Sb +5 was 73% with 0.80 mg/L MoO 3 /MoS 2-E composite after 28 min contacting time in the continuous studies (data not shown) in unilluminated conditions. As the MoO 3 /MoS 2-E composite concentration was increased from 0.01 mg/L up to 1.20 mg/L, the Sb +5 concentrations decreased to 2.0 mg/L with a Sb +5 yield of 80%. Table 1 showed the adsorption of Sb +5 and maximum adsorption capacity (q e ). As the MoO 3 /MoS 2-E composite concentration was increased up to 3.0 mg/L; the Sb +5 removal remained stable in unilluminated conditions. The maximum Sb +5 yield was obtained at 1.20 mg/L MoO 3 /MoS 2-E composite concentration. However, at high MoO 3 /MoS 2-E composite concentrations, after an optimum Sb +5 , the removal of Sb +5 decreased by the turbidity resulting in small amount of Sb +5 contacting with MoO 3 /MoS 2-E and low adsorption yields were detected. The mean diameter of MoO 3 / MoS 2-E composite generated under laboratory conditions was measured as 45 µm with an advanced light microscope. The batch studies performed with MoO 3 /MoS 2-E composites having low (30 µm) and high diameters (55 µm) did not exhibit significant differences in Sb +5 removal via adsorption process (82% and 83%) (data not shown).
By plotting of C e and C e /q e values obtained from the above table, it was found that the adsorption of Sb +5 on MoO 3 / MoS 2-E is suitable to Langmuir isotherm with q max = 0.625 (mg/g) and K values of 0.063 (L/mg) exhibiting a linear plot with r = 0.99 and regression coefficient of R 2 = 0.999 ( Figure 11).

Effect of increasing MoO 3 /MoS 2-E concentrations on photoelectrocatalytic removal at simulated sunlight powers
The preliminary studies showed that the maximum Sb +5 (10 mg/L Sb +5 ) photoelectrocatalytic yields (88%) was obtained with 1 mg/L MoO 3 /MoS 2-E composite at a sunlight power of 15 mW/m 2 after 24 min irradiation time in the continuous  studies (data not shown). As the MoO 3 /MoS 2-E composite concentration was increased from 0.01 mg/L up to 1.20 mg/L, the Sb +5 concentrations decreased to 0.70 mg/L with a Sb +5 yield of 93% ( Table 2). As the MoO 3 /MoS 2-E composite concentration was increased up to 6.0 mg/L, the Sb +5 removal remained stable. The max Sb +5 yields were obtained at 1.20 mg/L MoO 3 / MoS 2-E composite concentration. The increase of Sb +5 yield versus increasing MoO 3 /MoS 2-E composite is the production of numerous active sites, and resulting in increasing of hydroxyl radical production during photooxidation. However, at high MoO 3 /MoS 2-E composite concentrations, after an optimum Sb +5 photo electrocatalytic level, the removal of Sb +5 decreased by the turbidity resulting in small amount of sunlight power contact and low photoelectrocatalytical efficiencies [18]. The mean diameter of MoO 3 /MoS 2-E composite generated under laboratory conditions was measured as 45 micrometer with an advanced light microscope. The batch studies performed with MoO 3 /MoS 2-E composites having low (30 micrometer) and high diameters (55 µm) did not exhibit significant differences in Sb +5 removal (92% and 91%) (data not shown). The Sb +5 removal efficiency increased from 20% to 93% when the MoO 3 /MoS 2-E composite concentration was increased from 0.01 mg/L to 1.20 mg/L. A further increase in the MoO 3 /MoS 2-E composite (3 mg/L) resulted in a stable level of Sb +5 removal yields. At the beginning, the increase is due to the increased surface area and to the numerous suitable phtocatalytic activated points in the surface MoO 3 /MoS 2-E composite. At high MoO 3 /MoS 2-E composite concentrations such as 6 mg/L, the negative charge of the of Sb +5 was agglomerated. At high Sb +5 concentration like 1.20 mg/L, the activated surface regions of MoO 3 /MoS 2-E composite sites charged excessively and produced OH radicals were at high level. Under these conditions, the activated surface regions can regenerate electrons fully and they do not lose their activity [39]. Nineteen milligrams per liter Cu +2 and 21 mg/L As +3 present in petrochemical industry wastewater were removed via photocatalysis under similar operational conditions with photodegradation yields of 87% and 86.5%. This showed that MoO 3 /MoS 2 -E composite can be used as an excellent photocatalyst to remove the heavy metals from the industrial wastewaters.

Photoelectrocatalytic effects of increasing MoO 3 concentrations in MoO 3 /MoS 2-E on Sb +5 yields
As the MoO 3 concentrations were increased from 0.05 mg/L up to 0.1 mg/L, the Sb +5 yields increased from 45% up to 92% under sunlight (Table 3). Further increase of MoO 3 to 0.15 mg/L and to 0.30 mg/L did not improve the Sb +5 removal concentration. At these MoO 3 levels, the photoelectrocatalytic removal of Sb +5 occurred via electron transferring of the electrodes under sunlight irradiation. The results showed that further increase of MoO 3 concentration to 0.15 and to 0.30 mg/L affect negatively the Sb +5 yields. The reason for this is that the optimum amount of MoO 3 increases the number of active sites on the photocatalyst surface, which, in turn, increase the number of hydroxyl radicals (OH • ). However, excessive amounts of MoO 3 can retard the photocatalysis process because of excess amount of dopants, resulting in same Sb +5 yields. The chemicals used in the preparation of the MoO 3 /MoS 2-E was combined as low as to determine the optimum operational conditions to reach maximum removal both for adsorption and photooxidation processes under steady-state conditions. In these removal mechanisms, it is important to use the minimal concentrations and ratios of the reagents to develop the nanoadsorbent or nanocomposite. A cost analysis was performed to determine the MoO 3 /MoS 2-E spent. In order to remove 45 mg/L Sb +5 from 1 m 3 petrochemical wastewater via photocatalysis under sun light, the cost was found as 0.6 Euro (data not shown).

The effect of photooxidation times on the photoelectrocatalytic removal of 10 mg/L Sb +5
The maximum Sb +5 yield was obtained as 93% after 20 min photoelectrocatalytic contacting time for the operational conditions given above ( Table 4). The lowest Sb +5 photoelectrocatalytic yield (30%) was obtained at 5.0 min photooxidation time. An increment of time from 5.0 min up to 20 min increases the Sb +5 removal efficiency since the number of active places increases the removal yield of Sb +5 . Figure 11. Langmuir isotherm by plotting C e and C e /q e values obtained from the adsorption study (q max = 0.625 (mg/g) and K = 0.063 (L/mg)).

Effect of simulated sunlight powers on the removal Sb +5 concentration at 1.20 mg/L MoO 3 /MoS 2-E composite
As shown in Table 5, an increase in sunlight power from 2 mW/m 2 to15 mW/m 2 and to 26 mW/m 2 , the removal efficiency of Sb +5 increased from 70% to 88% and to 98%. The electron production rate varied depending on increasing sunlight powers. At high solar power, higher electron conduction occurred [40]. More electron carrying Sb +5 can be photooxidated easily. The effect of sunlight power on the removal of the Pb +5 showed that the solar power was very effective in the treatment of Sb +5 ( Table 5). The studies under UV exhibited superior performances (data not shown) since the electrons from UV exhibited higher entrance on the surface of the MoO 3 /MoS 2 composite as reported by Prado et al. [41].   Figure 12 shows the photocatalysis results at increasing Sb +5 and nanocomposite doses. The maximum Sb +5 photooxidation yield (95%) was observed at 10 mg/L Sb +5 concentration where the MoO 3 /MoS 2-E concentration was optimum (1.20 mg/L) at all studies of this research. At very high Sb +5 concentrations (25 mg/L), the photooxidation rates decreased since the acidified groups OH• ions decreased during photooxidations as reported by Qi and Pichler (2016) [42]adsorption behavior and other chemical properties are similar to that of arsenic (As.

Effect of pH on the photooxidation Sb +5 at 1.20 mg/L MoO 3 /MoS 2-E concentration
In this step of the study, in order to detect the influence of pH on photooxidation yields of Sb +5 , studies were performed at six pH values (4, 5, 6, 7, 8, and 9). As shown in Table 6, during the photooxidation of Sb +5 , the yield elevated with rising of pH. The removal efficiency of Sb +5 was measured as 45% at pH = 4 after 20 min contacting time. As the pH was increased to 5, no significant increase in Sb +5 removal was detected. The Sb +5 photooxidation yields was detected as 47% at this pH after 20 min. Then the pH increased to 6 and 9, respectively. The Sb +5 photooxidation yields were measured as 72% and 74%, respectively, in the aforementioned pH levels. The maximum Sb +5 photooxidation yield was detected as 81% after 20 min at pH = 8. The photoremoval of Sb +5 increased immediately after 10 min and reached steady state after 20 min at pH = 9 with a maximum Sb +5 photodegradation yield of 97%. Under alkaline conditions, the Sb +5 photoremoval is better than a neutral and acidic condition as reported by Qi and Pichler [42]adsorption behavior and other chemical properties are similar to that of arsenic (As. With increasing pH, the Sb +5 mainly exists in negative form, and the electron intensity of Sb +5 increased. This is the recommendation for the formation of reactive oxidative species resulting in increased photodegradation. O 2 and O 2 − radical species were generated. The deprotonated Sb +5 absorbed photons and to be excited to a triplet [43]adsorption behavior and other chemical properties are similar to that of arsenic (As. Since point zero charge of the MoO 3 /MoS 2-E composite catalyst surface was measured as 8.9, the photodegradation rates decreased under acidic and neutral pH as performed by Li et al. [adsorption behavior and other chemical properties are similar to that of arsenic (As9].
Usually, the surface of MoO 3 /MoS 2-E is negatively charged and the primary mechanism in adsorption could be ion exchange which happens between Sb +5 ions and the cations on MoO 3 /MoS 2-E surface [44][45][46][47]. Due to the stronger electrostatic attraction, Sb +5 could first approach MoO 3 /MoS 2-E surface and replace H + , and then form a Pb-S complexation with one or two S atoms, depending on the abundance of Sb +5 ions [47]. The formation of Sb 2 S 5 complexation is supported by the reaction generated in the surrounding of MoO 3 /MoS 2-E [48]. At the top layer of MoO 3 /MoS 2-E , each S atom possesses a tetrahedral electron configuration because of sp3 hybridization. Three of the sp3 orbitals form MoO 3 /MoS 2-E -S bonds while the fourth is occupied by a pair of electrons [49][50]. In this theory, Sb +5 can accept the ion pair electrons on MoO 3 / MoS 2-E surface due to its empty 6p orbitals and form stable coordinate covalent bonds [51]. Similarly, both electrostatic attraction and Sb +5 -sulfur complexation contribute to the adsorption of Sb +5 ions on s MoO 3 /MoS 2-E surface.

The effect of HCO 3
− on photoelectrocatalytical removal of Sb +5 HCO 3 − is a rich anion in water and does not absorb light in solar irradiation. However, it can react with strongly oxidizing radicals such as OH•, SO 4 −2 to produce some carbonate radicals (CO 3 −2 ). Also, adding HCO 3 sometimes elevates the pH of the medium. This decreases the photochemical activity of the some metals [26]whereas hydroxyl radicals (HO. The pK a1 and pK a2 of H 2 CO 3 are 6.39 and 10.38, respectively [43]whereas hydroxyl radicals (HO. At neutral pH, HCO 3 − ratio was measured as 78%, while 22% of the HCO 3 is measured as carbon dioxide dissolved in wastewater. Addition of bicarbonate (between 0.0 and 0.9 mg/L HCO 3 − ) did significantly affect the removal yields of Sb +5 at investigated pHs (7.0 and 8.0) [52]  and investigates its adsorptive behaviors toward Sb(III. The Sb +5 photoelectrocatalytical removal elevated after 20 min of irradiation time when HCO 3 − doses were increased from 0.0 mg/L to 0.9 mg/L at pH 7.0 and 8.0 ( Figure 13). Meanwhile, it is important to note that Sb +5 removal increased significantly at pH 8.0 when the HCO 3 − dose was increased from 0.0 mg/L to 0.9 mg/L. This change can be attributed to high bicarbonate concentrations releasing selective CO 3 −2 [53,54].  [55,56]. However, Zhu, on the contrary, proved that the presence of SO 4 2− fully supports Sb +5 adsorption on zerovalent iron [57]. Zhang et al. found that SO 4 2− does not allow Sb +5 adsorption on zerovalence iron [58]. Besides, Hu et al. also discovered that Sb +5 adsorption on Fe-Cu binary oxide is more affected by SO 4 2− than Sb +3 adsorption [59]. PO 4 3− is known to have an inhibitory effect on the adsorption of Sb(VI) and Sb(III) on ferric adsorbents. No study was found investigating the effects of the aforementioned ions on Sb +5 photooxidation yields in the presence of MoO 3 /MoS 2-E composite. The effects increasing of some ion concentrations on the of 10 mg/L Sb +5 at 1.20 mg/L MoO 3 /MoS 2-E composite concentration at 0.10 mg/L MoO 3 concentration after 20 min photooxidation of Sb +5 were tabulated in Table 7 for pH 6 and 9.

Effect of NO
The results showed that the PO 4 3− , SiO 3 2− ions increased the photocatalytic removal of Sb +5 at all concentrations for both pH values. Probably these ions did not compete with Sb +5 for adsorption sites on the surface of MoO 3 /MoS 2-E composite at the beginning of photocatalytic oxidation process and this phenomenon improve the scavenging effects of these anions by increasing the OH radical production as reported by Santiago et al. [60]. On the other hand, as reported by Jianhong et al., it is possible to distinguish between inner-sphere and outer-sphere anion surface complexes. The inner-sphere complex is stronger than the outer-sphere complex and was not dependent on the ionic strength in the presence of PO 4 3− , SiO 3 2− ions [60]. When the PO 4 3− , SiO 3 2− ion concentrations increased to 0.5 and 0.7 M at pH = 9.0, the Sb +5 might only form strong bond with MoO 3 /MoS 2-E and the adsorption of Sb + on MoO 3 /MoS 2-E was increased by the changes in ionic strength as reported by Duo [61]. By contrast, the presence of NO 3 − , Cl − , CO 3 2− , and SO 4 2− anions decrease the Sb +5 photooxidation yields due to nonscavenging effects of these anions by decreasing the OH radical production as reported by Santiago et al. [60]. The inhibition, promotion, competition, and synergistic interaction of coexisting/competing ions during Sb +5 photooxidation require further advanced research. These ions can compete for sorption of Sb +5 to outer MoO 3 /MoS 2-E surface at pH = 9.0 [61,62]. Recent studies have shown that mineralization as well as disinfection of HCO 3− , ClO 4− , NO 3 − , SO 4 2− and ions limits the activity of photocatalysts surface [63]. The effect of catalyst surface contamination with inorganic ions on photoactivity of TiO 2 is explained using several mechanisms [64]. They include adsorption competition of photons at active sites and on the particle surface, direct interaction with the photocatalyst, scavenging of radicals and electron gaps, and the scanning effect of UV rays. Actually, these properties are displayed by SO 4 2− , NO 3 − , Cl, HCO 3− , and PO 4 3− ions. The mechanism of the photocatalysis inhibition by Cland HCO 3 anions by scavenging of electron gaps and radicals are given by the reaction equations (3) and (4) [65].
Cl -+ OH• → Cl• + OH -(3) Cl -+ h + → Cl• (4) Furthermore, Clions cause inhibition of photocatalysis by TiO 2 , which can be explained by the preferential adsorption mechanism against the surface-bound OHions. While reducing the amount of OHions present on the TiO 2 surface, substitution Clions cause increased recombination of electron-electron gap pairs [64][65][66]. The range of ionic Sb +5 species stabilizes as the pH changes, causing changes in adsorption behavior of Sb +5 . The low adsorption of Sb +5 is likely due to competition for adsorption sites between hydroxyl ions, and for pH 4 to 6, Sb +5 is the [Sb (OH) 6− ] species [67]. For this reason, it is certain that the optimum pH value for adsorption at the initial stage of the photocatalytic mechanism is related to the zero charge point of MoO 3 /MoS 2-E in solution. The protonation reaction on the surface of the MoO 3 /MoS 2-E composite causes the surface to become positively charged, which ultimately increases the adsorption of Sb +5 ions due to the higher concentration of H + ions in the reaction mixture, promoting the removal of Sb +5 . On the contrary, if the pH is lower and higher than the zero charge pH, the MoO 3 /MoS 2-E composite surface and the adsorbate species will be negatively charged. Thus, the adsorption performance is affected through the process of electrostatic repulsion [68,69]. In order to obtain good removal efficiency of Sb +5 , the pH value should be kept below the pH zpc of the iron adsorbents within the pH range of 8-9. For Sb +3 , the neutral H 3 SbO 3 species dominates over a wide pH range (2-10) [67]. Thus, a change in pH value has little influence on the removal efficiency of Sb +3 compared to Sb +5 .

Discussion
The photocatalytic studies performed by some nanocatalysts showed that the removal of some heavy metals can be performed by a quick and short adsorption stage ending with photocatalytic reductions. In this study, both adsorption and photocatalytic removal of Sb + 5 on MoO 3 /MoS 2-E were studied. Since the photocatalytic yield of Sb + 5 is higher compared to adsorption capacity, detailed studies were performed with only photooxidation. Wang et al. found an adsorption of Ag + on individual MoS 2 surface and the other is the redox reaction that forms the segregated micrometer-sized metallic silver particles [70]. In this study, it was also found that some heavy metals like Ag +1 , Au 3+ , and Hg 2+ can be removed with a reduction−oxidation (redox) reaction between ionic metal species and MoS 2 from water by two-dimensional MoS 2 nanosheets suspended in aqueous solution [70]. Light also has been found to be an indispensable catalyst in the redox reaction of reduced metallic particles. Saadati et al. found that single-layer MoS 2 -MoO 3 heterojunction nanosheets with lateral average dimensions of ~70 nm showed an excitation-dependent behavior along with visible-light-induced photocatalytic activity [10]. The existence of MoO 3 domains with a broad light absorption due to the oxygen vacancies promoted the essential photocatalytic activity of single-layer MoS 2 sheets by facilitating separation of photoexcited electron-hole pairs and preventing their fast recombination. Thus, eventually the photocatalytic activity was enhanced. The scavenging feature of oxidizing radical species given by the NO 3 - , SO 4 2-, ClO 4 -, and HCO 3 ions were also responsible for reducing the rate of degradation. NO 3 ion may block the active sites of the catalysts by decreasing the photoactivity of the MoO 3 /MoS 2 composite. The aforementioned ions deactivate the catalyst due to the leaching of anions resulting in lack of formation of e -/h + pairs during photocatalysis. Because the surface of MoO 3 /MoS 2 is negatively charged, it interacts with positively charged heavy metals through electrostatic interaction. MoO 3 /MoS 2 exhibits high adsorption ability; as a results of initial fast adsorption process of Sb +5 , it was localized on active sites of MoO 3 /MoS 2 nanocomposite. This process results in high photocatalytic degradation. Due to forming core-shell structure, the light absorption of MoO 3 /MoS 2 sample in ultraviolet and visible regions increased significantly. As the result, MoO 3 /MoS 2 composite displays excellent photocatalytic activity under simulated sunlight irradiation. Since no data with Sb +5 yield was detected in the recent literature using MoO 3/ MoS 2-E composite, the adsorption and photocatalytic yields of the other metals, dyes, and organochlorinated compounds were correlated with our removal. As aforementioned in the introduction section, the Sb +5 adsorption and photooxidatıon yields found with this study were relatively higher than Pb +2 , Au +3 , Pb +2 , Cd +2 2, 4, 6-trichlorophenol, RhB a methylene blue dyes [4,5,[10][11][12][13][14][15][16][17].

Conclusion
Since Sb +5 and its compounds are considered to be the first pollutants in aquatic environments in the world and they are found at high levels in water, the treatment of Sb-contaminated wastewater has become more important today. Therefore, in this study, the MoO 3 /MoS 2-E composite was produced under laboratory conditions to remove Sb +5 from a crude petrochemical wastewater by the photoelectrocatalysis and adsorption process. MoO 3 /MoS 2-E composite was generated under laboratory conditions to remove Sb +5 from a raw petrochemical wastewater via photoelectrocatalysis process. An Ag/AgCl in saturated KCl, and a Pt electrode was found to be very effective in the removal of Sb +5 . pH is a very important factor in the photocatalysis of Sb +5 . For maximum removal of 10 mg/L initial Sb +5 concentration (E = 93%), the optimized operational conditions were as follows: 1.20 mg/L MoO 3 /MoS 2-E composite concentration, 0.10 mg/L MoO 3 concentration, 20 min photoelectrocatalysis time, at pH = 9, at HCO 3 -, PO 4 −3 , and SiO 3 -2 concentrations of 0.7 and 0.9 mg/L, respectively, at pH = 9.0 and 6.0, respectively, and at a simulated sunlight power of 15 mW/m 2 . The adsorption yield was found to be lower (80%) for the aforementioned operational conditions compared to photocatalysis.